move virtualenv realated functionality from Runner into plugins.VirtualEnv
plugins.VirtualEnv -> plugins.PythonDependencies
move Runner's script execution into new plugins.Execute
using yaml.safe_load
Script.name
check on script name moved to Script
remove script_name parameter (it is available via script.name)
remove class Runner
replay.runner -> replay.context
remove Runner
simplify/clarify test fixture parameter names